Iridescent coatings are useful in a diverse range of currently commercially significant products. These include credit cards, membership materials, board laminates, labels, toys, packaging materials, holographic imaging, and advertising displays to name a few examples.
Various techniques have been developed to produce iridescent effects for decorative and functional purposes on different articles of manufacture. These methods are highly specific in applicability to narrow ranges of chemical compositions and complex physical structures.
U.S. Pat. No. 3,235,399 discloses a method of creating a nacreous effect on polyvinyl chloride (“PVC”) by coating the PVC with a 0.1 to 1.5 wt % dispersion of mica flake in PVC and then solidifying the dispersion.
U.S. Pat. No. 3,766,061 teaches a procedure for generating striking optical properties for decorative uses by coating an object with a mixture of ester derivatives of cholesterol or cholesterol-like sterols.
U.S. Pat. No. 6,018,902 discloses a method of making iridescent reflective fishing lures by applying an optical stack of a plurality of dielectric film layers on a base layer. The optical stack comprises an odd number of layers numbering at least three. The first, third, etc. layers comprise the same reflective material which differs from the reflective material of the second, fourth, etc. layers.
A different technique known in the art involves forming an iridescent coating that comprises a textured base layer with a light diffracting surface contour. That is, the surface defines many micro-size impressions, usually grooves, in the thousands per centimeter peak-to-peak spacing range. Often the base layer is polymeric. To create an enhanced, high contrast iridescence on the polymer, the textured base layer surface can be metalized with an ultra-thin layer of a reflective metal.
Traditionally the grooves are formed by a mechanical technique, such as embossing. The base layer is impressed by a tool bearing an inverted, negative relief, groove profile. Under conditions of heat and pressure the positive relief groove profile is transferred to the base layer. Usually the impressions are formed by passing the base layer through the nip between a precisely machined embossing roll and a back-up tool. Embossing rolls with very fine negative relief groove profiles can be difficult and expensive to manufacture and are delicate once made. Moreover, the act of embossing grooves into the base layer adds an extra procedural step to the iridescent film production process.
It is desirable to have a method of making an iridescent film that is suitable for use with a broad range of base layer compositions and operating conditions. An iridescent film fabrication procedure that avoids the need to utilize many different layers is also desired. There is a need to produce an iridescent grooved surface on a polymer film without an extra, mechanical embossing step. Such non-mechanical groove-forming method beneficially would also eliminate the need to provide an expensive, precisely machined embossing roll. A procedure for making iridescent film that can be carried out on existing equipment without significant mechanical modifications or material changes is very much wanted.